Full-duplex communication links, a diagrammatic illustration of an individual one of which is shown in FIG. 1, are commonly employed in a wide variety of communication systems. In a digital communication system the format of the signal being conveyed over the link is typically an analog waveform the level of which conveys digital logic level information to the link users in some methodical manner. On occasion, as in the case of a routine maintenance operation, it may be necessary to couple signal measuring equipment to the link. If, for example, a signal monitoring device, such as an oscilloscope, is coupled to link 13 while only user A is transmitting, then the data waveform for user A would be observed on the scope display screen. Likewise, if only user B was transmitting, then the observed waveform would be that belonging to user B.
Because a full-duplex data link is one in which dual direction digital communications take place simultaneously between a user A at a `west` station 11 and a user B at an `east` station 12 at the opposite end of a link 13, then whatever device is used to monitor the signals must be able to both intercept the two signals and separate them from one another. Once the two signals are separated, they can be processed (via a demodulator) to recover the digital information conveyed from user A and/or user B.
Operationally, each user is able to recover the other's data through waveform separation resulting from an a priori knowledge of the transmitted waveform. Techniques used to accomplish this include transformer hybrids and digital echo cancellation. (It is assumed that the demodulators contain the necessary signal processing hardware and employ appropriate signal processing techniques to compensate for any distortion present in the separated signals due to the two wire transmission line.) In the case of merely coupling an oscilloscope to the link, when both user A and B are transmitting, the observed waveform on the link is the composite waveform for A and B, so that the two signals interfere with each other in such a manner that, in general, it is not possible to distinguish the signal transmitted from either user A or user B.
Historically, this inability to distinguish between the two signals has involved the insertion of some form of device directly into the link, as diagrammatically illustrated in FIG. 2. Unfortunately, such an installation first requires that the line be cut and that termination devices, such as respective type A and type B modems 21 and 22 diagrammatically illustrated in FIG. 3, be physically installed, thereby disrupting the ongoing communications between users A and B. Moreover, a power failure in such cascaded back-to-back modem equipment would interrupt link service.
Another technique comprises establishing an amplitude difference between the two signals Sa and Sb, by installing an attenuator in the line and measuring the amplitude of the signal on one side of the attenuator versus the signal level on the other side of the attenuator. Using this signal level information, user A's signal can be separated from user B's signal. Again, however, this scheme requires cutting the wire and installing the attenuator. Moreover, the permanent insertion of the attenuator imparts an addition loss that can degrade communications between users A and B.
A further approach involves sampling the wireline signals at two spaced apart locations along the link in order to establish a phase or amplitude difference between the sampled signals and thereby differentiate between signals Sa and Sb. However, due to the speed of propagation of the electrical signals on the link with respect to the data rate of the digital data, the separation distance between the two different tap points would have to be hundreds of feet, so that such a technique is not a practical solution to the problem.